> The default action, if not overridden by command line options, is to compare the file hierarchy rooted in the current directory against a specification read from the standard input. Messages are written to the standard output for any files whose characteristics do not match the specification, or which are missing from either the file hierarchy or the specification.

* https://www.freebsd.org/cgi/man.cgi?mtree

It can also be used to create a hierarchy:

* https://github.com/freebsd/freebsd/tree/master/etc/mtree